Porous media are bonded to the surface of a substrate for a wide variety of purposes. For many applications, such as the formation of a resilient or acoustically absorptive surface, neither the nature of the material which accomplishes the bond nor the depth to which the bond penetrates the porous facing is critical.
For a wide range of other applications, such as the purification of pharmaceutical fluids or the removal of bacteria from foods, e.g., milk and beer, bonded assemblies which include finely porous filter media secured to a solid substrate are used. Secure bonding of the porous medium to a solid substrate is particularly necessary when the porous medium is exposed during service to very high shear forces which would disrupt an unsupported membrane.
Filtration applications also typically require that the porous medium is bonded to the substrate such that the fluid passing through the membrane is provided with passageways through which it can flow as it issues from the membrane. Typically, the passageways are grooves cut or cast into a plane surface, the grooves being configured to drain collectively into a central outlet port, which the user connects to a receiver for the filtrate.
The porous medium may be secured to the substrate by applying a layer of a viscous adhesive to the substrate and then contacting the porous medium with the adhesive layer. The use of a third component which could leach into the filtrate, i.e. the adhesive, is very undesirable for many of the applications described above. In addition, the adhesive can often blind a substantial number of the pores and alter the permeability of the medium.
Bonded assemblies may also be produced by contemporaneously forming and integrally securing a porous medium to the surface of a substrate. This method, however, is severely limited by the requirement that the porous medium be precipitated from a liquid suspension and secured to the substrate in a single step. Some porous media, which may be employed effectively in filter applications, are not formed from liquid suspension. For example, polytetrafluoroethylene (e.g. Teflon.RTM. TFE) is typically made as a powder, which is then extruded to form a sheet, and the sheet is biaxially stretched to form a porous membrane.
A filter membrane may also be secured to a substrate by a method which involves the application of a solvent to which the filter membrane is inert, but which dissolves the substrate. The filter membrane is saturated with the solvent, and then contacted with the substrate. The contact of the saturated membrane with the substrate dissolves a portion of the substrate, which is then integrally secured to the membrane after the solvent is removed. This method has the severe fault that it may be extremely difficult to maintain a uniform distribution of solvent throughout the filter membrane at the time it is applied to the substrate. Simple dipping, or any procedure involving manipulation of the wet membrane, invariably leaves more solvent in some portions of the membrane than in others. As a result, an excessively thick bond may form in some areas of contact, while in other areas the bonding between the membrane and the substrate may be inadequate.
For many if not most applications, it is important that the membrane be positioned precisely on a specific location on the substrate. This is difficult to do, because the prewetted membrane quite generally is limp, i.e. has no rigidity, and this difficulty is compounded by the rapid evaporation of the solvent, such that a significant loss of solvent can occur in a few seconds.
Further, in the process described above, the solvent is typically allowed to evaporate during the dissolution and bonding process. The space within any grooves, which may be present in the substrate, is rapidly saturated by the vapor from a small fraction of the solvent and, thus, the bulk of the evaporation takes place at the exposed surface of the filter membrane. As solvent evaporates from the exposed surface, solvent from the remainder of the filter membrane migrates by capillarity through the membrane to the exposed surface. Accordingly, the solvent originally located in contact with the substrate, which contains dissolved substrate in solution, also evaporates from the exposed membrane surface. In the process, dissolved substrate may be deposited at the exposed surface of the filter membrane. This is highly undesirable, as the pores of the membrane may be at least partially clogged by the deposited substrate, locally altering the pore size and decreasing the permeability of the membrane.